There are three major technologies used for light bulbs today: incandescent, fluorescent and LED. Both incandescent and fluorescent bulbs have a well-defined end-of-life. This occurs when the filament breaks, and they cease producing light. Traditionally, for a plurality of light bulbs, “end-of-life” of both incandescent and fluorescent bulbs refers to the time when 50% of a population of them no longer emits light. In addition, because of the careful design of these bulbs, their end-of-life also corresponds to a specific level of light loss, which ensures that when the bulb needs to be replaced, it is at approximately the lowest acceptable level of light output.
However, because of the physical nature of the failure mode, there is naturally some distribution of failure times around this mean, which is convenient in application, as it means that not all the bulbs in a given install cycle fail simultaneously which might potentially cause under-lighting.
In the case of LED bulbs, failures are rare and occur after a very long time. Instead, the industry has chosen to define “end-of-life” for an LED bulb as the time at which it produces noticeably less light than initially, presently defined as 70% of initial lumens.
For many applications, the very long life time of LED bulbs is an advantage. Typically, most of these applications do not require replacing the bulb just because some degradation in light has occurred. However, there are other applications in which a minimum light level is required. For example, work and/or office areas can require a certain light level to ensure productivity, and/or a certain light level to avoid eye strain among office workers.
It can be appreciated that in circumstances such as these, it would be desirable to have the ability to turn off the LED light bulb when a specified level of light loss is reached. In addition, in order to avoid having all or a majority of light bulbs in an area turn off simultaneously, it would be desirable to have a variation and/or distribution of turn off times (i.e., end-of-life) of the light bulbs.